One-week exposure to crowding stress has been reported to affect body weight and pituitary-adrenal axis activity. This study has first evaluated whether the aforementioned changes are associated with altered exploratory activity. One-week crowding (16 rats/cage) diminished body weight gains, compared to the control situation (four rats/cage): actually, this difference was mainly due to a marked loss in food intake and body weight following the first day of crowding. On the other hand, 1-week crowded rats were not different from their controls when placed for the first time in an open field, as shown by the analysis of their exploratory activities, their number of grooming episodes, and their defecation. Psychoneuroendocrine effects of 1-day crowding were then investigated: actually, open field behaviors were not modified but basal plasma glucose levels were decreased. Moreover, neither plasma ACTH and corticosterone levels nor 2-min cold swim-induced increases in corticosterone levels were affected. Besides, cold swim-induced increases in plasma ACTH levels were amplified in crowded rats, and these increases were associated with hyperglycemia in crowded, but not in control rats. It is suggested that 1-day exposure to crowding has differential consequences upon feeding and exploratory behaviors while potentiating the responsiveness of stress hormones.